Gasket with positioning feature for clamped monolithic showerhead electrode

ABSTRACT

An electrode assembly for a plasma reaction chamber used in semiconductor substrate processing. The assembly includes an upper showerhead electrode which is mechanically attached to a backing plate by a series of spaced apart cam locks. A thermally and electrically conductive gasket with projections thereon is compressed between the showerhead electrode and the backing plate at a location three to four inches from the center of the showerhead electrode. A guard ring surrounds the backing plate and is movable to positions at which openings in the guard ring align with openings in the backing plate so that the cam locks can be rotated with a tool to release locking pins extending from the upper face of the electrode.

FIELD OF THE INVENTION

The invention relates to a showerhead electrode assembly of a plasmaprocessing chamber in which semiconductor components can bemanufactured.

SUMMARY

According to one embodiment, a gasket is provided for a showerheadelectrode assembly in which a monolithic stepped electrode is clamped toa backing plate and the showerhead electrode assembly comprises an upperelectrode of a capacitively coupled plasma processing chamber. Thestepped electrode is a circular plate having a plasma exposed surface ona lower face thereof and a mounting surface on an upper face thereof.The mounting surface includes a plurality of alignment pin recessesconfigured to receive alignment pins arranged in a pattern matchingalignment pin holes in a backing plate against which the plate is heldby cam locks and the plate includes process gas outlets arranged in apattern matching gas supply holes in the backing plate. The upper faceincludes a plurality of recesses which receive alignment features on thegasket. A plurality of circumferentially spaced apart pockets in anouter region of the mounting surface are configured to receive lockingpins therein adapted to cooperate with cam locks to clamp the steppedelectrode to the backing plate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-sectional view of a showerhead electrode assemblyforming an upper electrode of a capacitively coupled plasma reactor foretching substrates having a guard ring.

FIG. 2A is a three-dimensional representation of an exemplary cam lockfor clamping a stepped electrode in the reactor shown in FIG. 1.

FIG. 2B is a cross-sectional view of the exemplary cam lock electrodeclamp of FIG. 2A.

FIG. 3 shows side-elevation and assembly drawings of an exemplarylocking pin used in the cam lock clamp of FIGS. 2A and 2B.

FIG. 4A shows side-elevation and assembly drawings of an exemplary camshaft used in the cam lock clamp of FIGS. 2A and 2B.

FIG. 4B shows a cross-sectional view of an exemplary cutter-path edge ofa portion of the cam shaft of FIG. 4A.

FIG. 5A shows a showerhead electrode assembly with a stepped electrode,backing plate, thermal control plate, guard ring and top plate.

FIG. 5B shows a perspective view of the upper face of a modifiedshowerhead electrode and FIG. 5C shows a perspective view of the lowerface of a modified backing plate.

FIGS. 6A and 6B are perspective views of the stepped electrode of FIG.5A.

FIG. 7 is a perspective view of a backing plate of FIG. 5A.

FIG. 8 is a perspective view of the showerhead electrode assembly ofFIG. 5A without the guard ring.

FIG. 9 is a bottom view of a gasket according to a preferred embodiment.

FIG. 10 is a side view of the gasket shown in FIG. 9.

DETAILED DESCRIPTION

The fabrication of an integrated circuit chip typically begins with athin, polished slice of high-purity, single-crystal semiconductormaterial substrate (such as silicon or germanium) called a “wafer.” Eachwafer is subjected to a sequence of physical and chemical processingsteps that form the various circuit structures on the wafer. During thefabrication process, various types of thin films may be deposited on thewafer using various techniques such as thermal oxidation to producesilicon dioxide films, chemical vapor deposition to produce silicon,silicon dioxide, and silicon nitride films, and sputtering or othertechniques to produce other metal films.

After depositing a film on the semiconductor wafer, the uniqueelectrical properties of semiconductors are produced by substitutingselected impurities into the semiconductor crystal lattice using aprocess called doping. The doped silicon wafer may then be uniformlycoated with a thin layer of photosensitive, or radiation sensitivematerial, called a “resist.” Small geometric patterns defining theelectron paths in the circuit may then be transferred onto the resistusing a process known as lithography. During the lithographic process,the integrated circuit pattern may be drawn on a glass plate called a“mask” and then optically reduced, projected, and transferred onto thephotosensitive coating.

The lithographed resist pattern is then transferred onto the underlyingcrystalline surface of the semiconductor material through a processknown as etching. Vacuum processing chambers are generally used foretching and chemical vapor deposition (CVD) of materials on substratesby supplying an etching or deposition gas to the vacuum chamber andapplication of a radio frequency (RF) field to the gas to energize thegas into a plasma state.

A reactive ion etching system typically consists of an etching chamberwith an upper electrode or anode and a lower electrode or cathodepositioned therein. The cathode is negatively biased with respect to theanode and the container walls. The wafer to be etched is covered by asuitable mask and placed directly on the cathode. A chemically reactivegas such as CF₄, CHF₃, CC1F₃, HBr, Cl₂ and SF₆ or mixtures thereof withO₂, N₂, He or Ar is introduced into the etching chamber and maintainedat a pressure which is typically in the millitorr range. The upperelectrode is provided with gas hole(s), which permit the gas to beuniformly dispersed through the electrode into the chamber. The electricfield established between the anode and the cathode will dissociate thereactive gas forming plasma. The surface of the wafer is etched bychemical interaction with the active ions and by momentum transfer ofthe ions striking the surface of the wafer. The electric field createdby the electrodes will attract the ions to the cathode, causing the ionsto strike the surface in a predominantly vertical direction so that theprocess produces well-defined vertically etched sidewalls. The etchingreactor electrodes may often be fabricated by bonding two or moredissimilar members with mechanically compliant and/or thermallyconductive adhesives, allowing for a multiplicity of function.

FIG. 1 shows a cross-sectional view of a portion of a showerheadelectrode assembly 100 of a plasma processing system for etchingsubstrates. As shown in FIG. 1, the showerhead electrode assembly 100includes a stepped electrode 110, a backing plate 140, and a guard ring(or outer ring) 170. The showerhead electrode assembly 100 also includesa plasma confinement assembly (or wafer area pressure (WAP) assembly)180, which surrounds the outer periphery of the upper electrode 110 andthe backing plate 140.

The assembly 100 also includes a thermal control plate 102, and an upper(top) plate 104 having liquid flow channels therein and forming atemperature controlled wall of the chamber. The stepped electrode 110 ispreferably a cylindrical plate and may be made of a conductive highpurity material such as single crystal silicon, polycrystalline silicon,silicon carbide or other suitable material (such as aluminum or alloythereof, anodized aluminum, yttria coated aluminum). The backing plate140 is mechanically secured to the electrode 110 with mechanicalfasteners described below. The guard ring 170 surrounds the backingplate 140 and provides access to cam locking members as described below.

The showerhead electrode assembly 100 as shown in FIG. 1 is typicallyused with an electrostatic chuck (not shown) incorporating a flat lowerelectrode on which a wafer is supported at a distance of about 1 to 2 cmbelow the upper electrode 110. An example of such a plasma processingsystem is a parallel plate type reactor, such as the Exelan® dielectricetch systems, made by Lam Research Corporation of Fremont, Calif. Suchchucking arrangements provide temperature control of the wafer bysupplying backside helium (He) pressure, which controls the rate of heattransfer between the wafer and the chuck.

The upper electrode 110 is a consumable part which must be replacedperiodically. To supply process gas to the gap between the wafer and theupper electrode, the upper electrode 110 is provided with a gasdischarge passages 106, which are of a size and distribution suitablefor supplying a process gas, which is energized by the electrode andforms plasma in a reaction zone beneath the upper electrode 110.

The showerhead electrode assembly 100 also includes a plasma confinementassembly (or wafer area plasma (WAP) assembly) 180, which surrounds theouter periphery of the upper electrode 110 and the backing plate 140.The plasma confinement assembly 180 is preferably comprised of a stackor plurality of spaced-apart quartz rings 190, which surrounds the outerperiphery of upper electrode 110 and the backing plate 140. Duringprocessing, the plasma confinement assembly 180 causes a pressuredifferential in the reaction zone and increases the electricalresistance between the reaction chamber walls and the plasma therebyconfining the plasma between the upper electrode 110 and the lowerelectrode (not shown).

During use, the confinement rings 190 confine the plasma to the chambervolume and controls the pressure of the plasma within the reactionchamber. The confinement of the plasma to the reaction chamber is afunction of many factors including the spacing between the confinementrings 190, the pressure in the reaction chamber outside of theconfinement rings and in the plasma, the type and flow rate of the gas,as well as the level and frequency of RF power. Confinement of theplasma is more easily accomplished if the spacing between theconfinement rings 190 is very small. Typically, a spacing of 0.15 inchesor less is required for confinement. However, the spacing of theconfinement rings 190 also determines the pressure of the plasma, and itis desirable that the spacing can be adjusted to achieve the pressurerequired for optimal process performance while maintaining plasma.Process gas from a gas supply is supplied to electrode 110 through oneor more passages in the upper plate 104 which permit process gas to besupplied to a single zone or multiple zones above the wafer.

The electrode 110 is preferably a planar disk or plate having a uniformthickness from center (not shown) to an area of increased thicknessforming a step on the plasma exposed surface extending inwardly from anouter edge. The electrode 110 preferably has a diameter larger than awafer to be processed, e.g., over 300 mm. The diameter of the upperelectrode 110 can be from about 15 inches to about 17 inches forprocessing 300 mm wafers. The upper electrode 110 preferably includesmultiple gas passages 106 for injecting a process gas into a space in aplasma reaction chamber below the upper electrode 110.

Single crystal silicon and polycrystalline silicon are preferredmaterials for plasma exposed surfaces of the electrode 110. High-purity,single crystal or polycrystalline silicon minimizes contamination ofsubstrates during plasma processing as it introduces only a minimalamount of undesirable elements into the reaction chamber, and also wearssmoothly during plasma processing, thereby minimizing particles.Alternative materials including composites of materials that can be usedfor plasma-exposed surfaces of the upper electrode 110 include aluminum(as used herein “aluminum” refers to pure Al and alloys thereof), yttriacoated aluminum, SiC, SiN, and AlN, for example.

The backing plate 140 is preferably made of a material that ischemically compatible with process gases used for processingsemiconductor substrates in the plasma processing chamber, has acoefficient of thermal expansion closely matching that of the electrodematerial, and/or is electrically and thermally conductive. Preferredmaterials that can be used to make the backing plate 140 include, butare not limited to, graphite, SiC, aluminum (Al), or other suitablematerials.

The upper electrode 110 is attached mechanically to the backing plate140 without any adhesive bonding between the electrode and backingplate, i.e., a thermally and electrically conductive elastomeric bondingmaterial is not used to attach the electrode to the backing plate.

The backing plate 140 is preferably attached to the thermal controlplate 102 with suitable mechanical fasteners, which can be threadedbolts, screws, or the like. For example, bolts (not shown) can beinserted in holes in the thermal control plate 102 and screwed intothreaded openings in the backing plate 140. The thermal control plate102 includes a flexure portion 184 and is preferably made of a machinedmetallic material, such as aluminum, an aluminum alloy or the like. Theupper temperature controlled plate 104 is preferably made of aluminum oran aluminum alloy. The plasma confinement assembly (or wafer area plasmaassembly (WAP)) 180 is positioned outwardly of the showerhead electrodeassembly 100. A suitable plasma confinement assembly 180 including aplurality of vertically adjustable plasma confinement rings 190 isdescribed in commonly owned U.S. Pat. No. 5,534,751, which isincorporated herein by reference in its entirety.

The upper electrode can be mechanically attached to the backing plate bya cam lock mechanism as described in commonly-owned U.S. applicationSer. No. 61/036,862, filed Mar. 14, 2008, the disclosure of which ishereby incorporated by reference. With reference to FIG. 2A, athree-dimensional view of an exemplary cam lock electrode clamp includesportions of an electrode 201 and a backing plate 203. The electrodeclamp is capable of quickly, cleanly, and accurately attaching aconsumable electrode 201 to a backing plate in a variety of fab-relatedtools, such as the plasma etch chamber shown in FIG. 1.

The electrode clamp includes a stud (locking pin) 205 mounted into asocket 213. The stud may be surrounded by a disc spring stack 215, such,for example, stainless steel Belleville washers. The stud 205 and discspring stack 215 may then be press-fit or otherwise fastened into thesocket 213 through the use of adhesives or mechanical fasteners. Thestud 205 and the disc spring stack 215 are arranged into the socket 213such that a limited amount of lateral movement is possible between theelectrode 201 and the backing plate 203. Limiting the amount of lateralmovement allows for a tight fit between the electrode 201 and thebacking plate 203, thus ensuring good thermal contact, while stillproviding some movement to account for differences in thermal expansionbetween the two parts. Additional details on the limited lateralmovement feature are discussed in more detail, below.

In a specific exemplary embodiment, the socket 213 is fabricated frombearing-grade Torlon®. Alternatively, the socket 213 may be fabricatedfrom other materials possessing certain mechanical characteristics suchas good strength and impact resistance, creep resistance, dimensionalstability, radiation resistance, and chemical resistance may be readilyemployed. Various materials such as polyamides, polyimides, acetals, andultra-high molecular weight polyethylene materials may all be suitable.High temperature-specific plastics and other related materials are notrequired for forming the socket 213 as 230° C. is a typical maximumtemperature encountered in applications such as etch chambers.Generally, a typical operating temperature is closer to 130° C.

Other portions of the electrode clamp are comprised of a camshaft 207surrounded at each end by a pair of camshaft bearings 209. The camshaft207 and camshaft bearing assembly is mounted into a backing plate bore211 machined into the backing plate 203. In a typical application for anetch chamber designed for 300 mm semiconductor wafers, eight or more ofthe electrode clamps may be spaced around the periphery of the electrode201/backing plate 203 combination.

The camshaft bearings 209 may be machined from a variety of materialsincluding Torlon®, Vespel®, Celcon®, Delrin®, Teflon®, Arlon®, or othermaterials such as fluoropolymers, acetals, polyamides, polyimides,polytetrafluoroethylenes, and polyetheretherketones (PEEK) having a lowcoefficient of friction and low particle shedding. The stud 205 andcamshaft 207 may be machined from stainless steel (e.g., 316, 316L,17-7, etc.) or any other material providing good strength and corrosionresistance.

Referring now to FIG. 2B, a cross-sectional view of the electrode camclamp further exemplifies how the cam clamp operates by pulling theelectrode 201 in close proximity to the backing plate 203. The stud205/disc spring stack 215/socket 213 assembly is mounted into theelectrode 201. As shown, the assembly may be screwed, by means ofexternal threads on the socket 213 into a threaded pocket in theelectrode 201. However, the socket may be mounted by adhesives or othertypes of mechanical fasteners as well.

In FIG. 3, an elevation and assembly view 300 of the stud 205 having anenlarged head, disc spring stack 215, and socket 213 provides additionaldetail into an exemplary design of the cam lock electrode clamp. In aspecific exemplary embodiment, a stud/disc spring assembly 301 is pressfit into the socket 213. The socket 213 has an external thread and ahexagonal top member allowing for easy insertion into the electrode 201(see FIGS. 2A and 2B) with light torque (e.g., in a specific exemplaryembodiment, about 20 inch-pounds). As indicated above, the socket 213may be machined from various types of plastics. Using plastics minimizesparticle generation and allows for a gall-free installation of thesocket 213 into a mating pocket on the electrode 201.

The stud/socket assembly 303 illustrates an inside diameter in an upperportion of the socket 213 being larger than an outside diameter of amid-section portion of the stud 205. The difference in diameters betweenthe two portions allows for the limited lateral movement in theassembled electrode clamp as discussed above. The stud/disc springassembly 301 is maintained in rigid contact with the socket 213 at abase portion of the socket 213 while the difference in diameters allowsfor some lateral movement. (See also, FIG. 2B.)

With reference to FIG. 4A, an exploded view 400 of the camshaft 207 andcamshaft bearings 209 also indicates a keying pin 401. The end of thecamshaft 207 having the keying pin 401 is first inserted into thebacking plate bore 211 (see FIG. 2B). A pair of small mating holes (notshown) at a far end of the backing plate bore 211 provide properalignment of the camshaft 207 into the backing plate bore 211. Aside-elevation view 420 of the camshaft 207 clearly indicates a possibleplacement of a hex opening 403 on one end of the camshaft 207 and thekeying pin 401 on the opposite end.

For example, with continued reference to FIGS. 4A and 2B, the electrodecam clamp is assembled by inserting the camshaft 207 into the backingplate bore 211. The keying pin 401 limits rotational travel of thecamshaft 207 in the backing plate bore 211 by interfacing with one ofthe pair of small mating holes. The camshaft may first be turned in onedirection though use of the hex opening 403, for example,counter-clockwise, to allow entry of the stud 205 into the camshaft 207,and then turned clockwise to fully engage and lock the stud 205. Theclamp force required to hold the electrode 201 to the backing plate 203is supplied by compressing the disc spring stack 215 beyond their freestack height. The camshaft 207 has an internal eccentric internal cutoutwhich engages the enlarged head of the shaft 205. As the disc springstack 215 compresses, the clamp force is transmitted from individualsprings in the disc spring stack 215 to the socket 213 and through theelectrode 201 to the backing plate 203.

In an exemplary mode of operation, once the camshaft bearings areattached to the camshaft 207 and inserted into the backing plate bore211, the camshaft 207 is rotated counterclockwise to its full rotationaltravel. The stud/socket assembly 303 (FIG. 3) is then lightly torquedinto the electrode 201. The head of the stud 205 is then inserted intothe vertically extending through hole below the horizontally extendingbacking plate bore 211. The electrode 201 is held against the backingplate 203 and the camshaft 207 is rotated clockwise until either thekeying pin drops into the second of the two small mating holes (notshown) or an audible click is heard (discussed in detail, below). Theexemplary mode of operation may be reversed to dismount the electrode201 from the backing plate 203. However, features such as the audibleclick are optional in the cam lock arrangement.

With reference to FIG. 4B, a sectional view A-A of the side-elevationview 420 of the camshaft 207 of FIG. 4A indicates a cutter path edge 440by which the head of the stud 205 is fully secured. In a specificexemplary embodiment, the two radii R₁ and R₂ are chosen such that thehead of the stud 205 makes the optional audible clicking noise describedabove to indicate when the stud 205 is fully secured.

FIG. 5A illustrates an upper electrode assembly 500 for a capacitivelycoupled plasma chamber which includes the following features: (a) acam-locked non-bonded electrode 502; (b) a backing plate 506; and (c) aguard ring 508 which allows access to cam locks holding the electrode tothe backing plate 506.

The electrode assembly 500 includes a thermal control plate 510 boltedfrom outside the chamber to a temperature controlled top wall 512 of thechamber. The electrode 502 is releasably attached to the backing platefrom inside the chamber by cam-lock mechanisms 514 described earlierwith reference to FIGS. 2-4.

In a preferred embodiment, the electrode 502 of the electrode assembly500 can be disassembled by (a) rotating the guard ring 508 to a firstposition aligning four holes in the guard ring with four cam locks 514located at spaced positions in the outer portion of the backing plate;(b) inserting a tool such as an allen wrench through each hole in theguard ring and rotating each cam lock to release a vertically extendinglocking pin of each respective cam lock; (c) rotating the guard ring 90°to a second position aligning the four holes in the guard ring withanother four cam locks; and (d) inserting a tool such as an allen wrenchthrough each hole in the guard ring and rotating each respective camlock to release a locking pin of each respective cam lock; whereby theelectrode 502 can be lowered and removed from the plasma chamber.

FIG. 5A also shows a cross-sectional view of one of the cam lockarrangements wherein a rotatable cam lock 514 is located in ahorizontally extending bore 560 in an outer portion of the backing plate506. The cylindrical cam lock 514 is rotatable by a tool such as analien wrench to (a) a lock position at which an enlarged end of alocking pin 562 is engaged by a cam surface of the cam lock 514 whichlifts the enlarged head of the locking pin or (b) a release position atwhich the locking pin 562 is not engaged by the cam lock 514. Thebacking plate includes vertically extending bores in its lower facethrough which the locking pins are inserted to engage the cam locks.

In the embodiment shown in FIG. 5A, an outer step in the backing plate506 mates with an annular recessed mounting surface on the upper face ofthe showerhead electrode 502. In an alternative arrangement, the stepand recess can be omitted such that the lower face of the backing plateand the upper face of the showerhead electrode are planar surfaces. FIG.5B shows a cross-section of a modified showerhead electrode 502A havinga flat upper surface 522A, five alignment pin holes 520A, eight pockets550A, gas holes 528A, and two recesses 520B for mating with projectionsof a gasket located between the third and fourth row of gas holes. FIG.5C shows a modified backing plate 506A having a planar lower surface522B, five alignment pin holes 520C, eight cam locks 514B, and annulargasket receiving surfaces G1 and G2.

FIGS. 6A-B show details of the electrode 502. The electrode 502 ispreferably a plate of high purity (less than 10 ppm impurities) lowresistivity (0.005 to 0.02 ohm-cm) single crystal silicon with alignmentpin holes 520 in an upper face (mounting surface) 522 which receivealignment pins 524. Gas holes 528 extend from the upper face to thelower face (plasma exposed surface) 530 and can be arranged in anysuitable pattern. In the embodiment shown, the gas holes are arranged in13 circumferentially extending rows with 3 gas holes in the first rowlocated about 0.5 inch from the center of the electrode, 13 gas holes inthe second row located about 1.4 inches from the center, 23 gas holes inthe third row located about 2.5 inches from the center, 25 gas holes inthe fourth row located about 3.9 inches from the center, 29 gas holes inthe fifth row located about 4.6 inches from the center, 34 gas holes inthe sixth row located about 5.4 inches from the center, 39 gas holes inthe seventh row located about 6 inches from the center, 50 gas holes inthe eighth row located about 7.5 inches from the center, 52 gas holes inthe ninth row located about 8.2 inches from the center, 53 gas holes inthe tenth row located about 9 inches from the center, 57 gas holes inthe eleventh row located about 10.3 inches from the center, 59 gas holesin the twelfth row located about 10.9 inches from the center and 63holes in the thirteenth row located about 11.4 inches from the center.

In an alternative arrangement, 562 gas holes can be arranged with 4holes in the first row located 0.25 inch from the center, 10 holes in asecond row located about 0.72 inch from the center, 20 holes in a thirdrow about 1.25 inches from the center, 26 holes in a fourth row about1.93 inches from the center, 30 holes in a fifth row about 2.3 inchesfrom the center, 36 holes in a sixth row about 2.67 inches from thecenter, 40 holes in a seventh row about 3.0 inches from the center, 52holes in an eighth row about 3.73 inches from the center, 58 holes in aninth row about 4.1 inches from the center, 62 holes in a tenth rowabout 4.48 inches from the center, 70 holes in an eleventh row about5.17 inches from the center, 74 holes in a twelfth row about 5.44 inchesfrom the center and 80 holes in a thirteenth row about 5.71 inches fromthe center.

In the embodiment shown in FIG. 5A, the upper face of the electrodeincludes 9 alignment pin holes with 3 pin holes near the center, 3 pinholes inward of the annular recess and 3 pin holes in the annular recessnear the outer edge of the electrode. The 3 central pin holes areradially aligned and include a pin hole at the center of the innerelectrode and 2 pin holes between the third and fourth rows of gasholes. The intermediate pin holes near the annular recess include onepin hole radially aligned with the central pin hole and two other pinholes spaced 120° apart. The outer 3 pin holes are spaced 120° apart atlocations between adjacent pockets.

FIG. 6A is a front perspective view showing the plasma exposed surface530 of the electrode 502 with the 13 rows of gas holes. FIG. 6B shows aperspective view of the upper face with the 13 rows of gas holes.

The electrode 502 includes an outer step (ledge) 536 which supports theguard ring 508, the upper face (mounting surface) 522 which engages alower surface of the backing plate 506, the lower face (plasma exposedstepped surface) 530 which includes inner tapered surface 544, ahorizontal surface 546, and an outer tapered surface 548 and 8 pockets550 in upper face 540 in which the locking pins are mounted.

FIG. 7 is a perspective view of backing plate 506. The backing plateincludes 13 rows of gas passages 584 which align with the passages 528in the showerhead electrode 502. The upper face 586 of the backing plateincludes three annular regions 588 a, 588 b, 588 c which contact annularprojections of the thermal control plate 510. The thermal control platecan be attached to the top wall of the plasma chamber by fastenersextending through the top wall into the thermal control plate asdisclosed in commonly-assigned U.S. Patent Publication Nos.2005/0133160, 2007/0068629, 2007/0187038, 2008/0087641 and 2008/0090417,the disclosures of which are hereby incorporated in their entirety.Threaded openings 590 are located in an outer periphery of the upperface 586 and the annular regions 588 a, 588 b, 588 c to receivefasteners extending through openings in the top plate 512 and thermalcontrol plate 510 to hold the backing plate 506 in contract with thethermal control plate 510. See, for example, commonly-assigned U.S.Patent Publication No. 2008/0087641 for a description of fasteners whichcan accommodate thermal cycling. A groove 592 in the upper face 586receives an O-ring which provides a gas seal between the backing plate506 and the thermal control plate 510. Alignment pin bores 594 in theupper face 586 receive alignment pins which fit into alignment pin boresin the thermal control plate. Horizontally extending threaded openings561 at positions between bores 560 receive dielectric fasteners used toprevent the guard ring from rotating and plug the access bores in theguard ring after assembly of the showerhead electrode.

FIG. 8 is a perspective view of the showerhead electrode assembly 500with the guard ring removed. As explained earlier, the guard ring can berotated to one or more assembly positions at which the cam locks can beengaged and rotated to a lock position at which dielectric fasteners canbe inserted into openings 561 to maintain the guard ring out of contactwith the outer periphery of the backing plate and thus allow for thermalexpansion of the backing plate. The thermal control plate includes aflange 595 with openings 596 through which actuators support the plasmaconfinement rings. Details of the mounting arrangement of plasmaconfinement ring assemblies can be found in commonly-assigned U.S.Patent Publication No. 2006/0207502 and 2006/0283552, the disclosures ofwhich are hereby incorporated in their entirety.

The mounting surface 522 of the electrode abuts an opposed surface ofthe backing plate 506 as a result of the clamping force exerted by the 8locking pins held by the 8 cam locks in the backing plate. The guardring 508 covers the mounting holes in the backing plate 506 and theaccess openings in the guard ring are filled with removable inserts madeof plasma resistant polymer material such as Torlon®, Vespel®, Celcon®,Delrin®, Teflon®, Arlon®, or other materials such as fluoropolymers,acetals, polyamides, polyimides, polytetrafluoroethylenes, andpolyetheretherketones (PEEK) having a low coefficient of friction andlow particle shedding.

With reference to FIG. 5A, electrical contact between the backing plate506 and electrode 502 is provided by one or more gaskets 556 such asannular sections of a suitable material such as “Q-PAD II” availablefrom the Bergquist Company. Such gaskets are located at the outerperiphery of the electrode and at one or more locations between thecentral alignment pin and the outer gasket. For example, annular gasketshaving diameters of about 4 and 12 inches can be used. Commonly-ownedU.S. application Ser. No. 11/896,375, filed Aug. 31, 2007, includesdetails of gaskets made of Q-PAD material, the disclosure of which ishereby incorporated by reference. To provide different process gasmixtures and/or flow rates, one or more optional gas partition seals canbe provided between the center alignment pin and the outer gasket. Forexample, a single O-ring can be provided between the electrode 502 andthe backing plate 506 at a location between the inner and outer gasketsto separate an inner gas distribution zone from an outer gasdistribution zone. An O-ring 558 located between the electrode 502 andthe backing plate 506 along the inner periphery of the outer gasket canprovide a gas and particle seal between the electrode and backing plate.

FIG. 9 shows a bottom view of a preferred gasket 900 having a pluralityof alignment features in the form of projections 902 on a lower surface904 thereof. The electrode 502A includes a plurality of recesses (520Bin FIG. 5B) sized to receive the projections on the gasket 900. In theembodiment shown, two projections 902 are located 180° apart and theprojections have identical cylindrical shapes which fit within roundrecesses 520B in the electrode 502A located between the third and fourthcircumferential rows of gas passages 528A. The projections arepreferably sized to be frictionally engaged in the recesses 520B in theelectrode 502A. While cylindrical projections having diameters greaterthan half the width of the gasket are shown in FIG. 9, the projectionscan have any desired shape and size and the number of projections can be3, 4, 5, 6, 7, 8 or more, if desired. For example, the gasket can be aflat ring of uniform thickness of under 0.01 inch and the projectionscan be at least 2, 3, 4 or 5 times thicker than the thickness of theflat ring. Although the projections could be formed by molding integralprojections or deforming portions of the flat ring into projections, itis preferred to form the projections from a different material having agreater thickness than the flat ring and attaching the projections tothe flat ring with adhesive compatible in a vacuum environment of aplasma processing chamber.

The gasket is preferably electrically and thermally conductive and madeof a material which preferably does not outgas in a high-vacuumenvironment, e.g., about 10 to 200 mTorr, has low particulate generationperformance; is compliant to accommodate shear at contact points; isfree of metallic components that are lifetime killers in semiconductorsubstrates such as Ag, Ni, Cu and the like. The gasket can be asilicone-aluminum foil sandwich gasket structure or anelastomer-stainless steel sandwich gasket structure. Preferably, thegasket is an aluminum sheet coated on upper and lower sides with athermally and electrically conductive rubber compatible in a high vacuumenvironment used in semiconductor manufacturing wherein steps such asplasma etching are carried out. The gasket is preferably compliant suchthat it can be compressed when the electrode and backing plate aremechanically clamped together but prevent opposed surfaces of theelectrode and backing plate from rubbing against each other duringtemperature cycling of the showerhead electrode.

The gasket 900 shown in FIG. 9 is preferably a laminate of electricallyand thermally conductive material (such as “Q-PAD” foil materialavailable from The Bergquist Company). The gasket 900 for the G1location in FIG. 5C preferably has an inner diameter of about 2.93inches, an outer diameter of about 3.43 inches and a thickness of about0.006 inch. This gasket has two projections 902 which comprise acylindrical piece of sheet material such as silicone rubber with adiameter of about 0.185 inch and height of about 0.026 to 0.034 inch.The projections 902 are preferably adhesively bonded to one side of thegasket 900 by suitable adhesive such as a silicone elastomer adhesive,e.g. RTV 3140 silicone adhesive available from Dow Corning. The gasketcan be made by cutting or stamping a ring out of a sheet of gasketmaterial. Likewise, the projections can be cut or stamped out of thesheet of the same or different material such as a resilient materialwhich may or may not be thermally and/or electrically conductive. Forexample, the projections can be of a rubbery material such as blacksilicone rubber which elastically deforms and frictionally engages therecesses in the showerhead electrode. The gasket 900 can thus be mountedon the showerhead electrode without adhesive to allow easy removal ofthe gasket during cleaning or replacement of the showerhead electrode.

While the invention has been described in detail with reference tospecific embodiments thereof, it will be apparent to those skilled inthe art that various changes and modifications can be made, andequivalents employed, without departing from the scope of the appendedclaims.

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 12. (canceled)
 13. (canceled)
 14. A method of treating asemiconductor substrate in a plasma chamber, said method comprising thesteps of: supporting the semiconductor substrate on a lower electrode inthe chamber; supplying process gas to the chamber; forming a plasmaadjacent an exposed surface of an upper electrode; and processing thesemiconductor substrate with the plasma; wherein the upper electrodecomprises a showerhead electrode of a showerhead electrode assemblycomprising: a backing plate including a plurality of cam locks locatedin an outer portion of the backing plate; a showerhead electrode havinga diameter of over 12 inches and including a central portion and aperipheral portion defined by upper and lower faces of the showerheadelectrode, the upper face including a planar surface extending acrossthe central portion, the lower face defined by a planar inner surfaceextending across the central portion and a stepped outer surfaceextending across the peripheral portion, the stepped outer surfaceincluding an annular planar surface defining an area of increasedthickness of the showerhead electrode; a plurality of gas outlets in thecentral portion of the electrode through which process gas can bedelivered to a gap between the showerhead electrode and a lowerelectrode on which a semiconductor substrate is supported, a pluralityof circumferentially spaced apart pockets in the peripheral portion ofthe upper face, the pockets supporting upwardly extending locking pinsengaged with the cam locks of the backing plate so as to clamp theshowerhead electrode to the backing plate; and a gasket comprising anannular strip of thermally and electrically conductive material and aplurality of projections on a surface thereof, the annular strip havingan outer diameter smaller than an outer diameter of the showerheadelectrode, and each of the projections having a height at least twotimes greater than the thickness of the annular strip compressed betweenthe showerhead electrode and the backing plate, the projections of thegasket located in recesses in the showerhead electrode and the annularstrip preventing the upper face of the showerhead electrode from rubbingagainst a lower face of the backing plate during temperature cycling ofthe showerhead electrode.
 15. The method as claimed in claim 14, whereintemperature of the showerhead electrode is controlled by a temperaturecontrolled top wall of the chamber, a thermal control plate and thebacking plate, the thermal control plate including annular projectionsforming plenums between the thermal control plate and the backing plate,the plenums in fluid communication with gas passages in the backingplate aligned with the gas passages in the showerhead electrode, thebacking plate providing a thermal path between the showerhead electrodeand the thermal control plate.
 16. The method as claimed in claim 14,wherein the semiconductor substrate comprises a semiconductor wafer andthe processing step comprises etching the semiconductor wafer with theplasma.
 17. The method as claimed in claim 14, wherein the upperelectrode is grounded and the bottom electrode is powered during theprocessing step.
 18. The method as claimed in claim 14, comprisingheating the showerhead electrode and backing plate to an elevatedtemperature which causes differential thermal expansion of theshowerhead electrode and the backing plate, and accommodating thethermal expansion by movement of the locking pins while the gasketprevents rubbing of the backing plate against the showerhead electrode.19. A method of replacing a showerhead electrode of a showerheadelectrode assembly comprising: a backing plate including a plurality ofcam locks located in an outer portion of the backing plate; a showerheadelectrode having a diameter of over 12 inches and including a centralportion and a peripheral portion defined by upper and lower faces of theshowerhead electrode, the upper face including a planar surfaceextending across the central portion, the lower face defined by a planarinner surface extending across the central portion and a stepped outersurface extending across the peripheral portion, the stepped outersurface including an annular planar surface defining an area ofincreased thickness of the showerhead electrode; a plurality of gasoutlets in the central portion of the electrode through which processgas can be delivered to a gap between the showerhead electrode and alower electrode on which a semiconductor substrate is supported, aplurality of circumferentially spaced apart pockets in the peripheralportion of the upper face, the pockets supporting upwardly extendinglocking pins engaged with the cam locks of the backing plate so as toclamp the showerhead electrode to the backing plate; and a gasketcomprising an annular strip of thermally and electrically conductivematerial and a plurality of projections on a surface thereof, theannular strip having an outer diameter smaller than an outer diameter ofthe showerhead electrode, and each of the projections having a height atleast two times greater than the thickness of the annular stripcompressed between the showerhead electrode and the backing plate, theprojections of the gasket located in recesses in the showerheadelectrode and the annular strip preventing the upper face of theshowerhead electrode from rubbing against a lower face of the backingplate during temperature cycling of the showerhead electrode, comprisingreleasing the cam locks to disengage the cam locks from the lockingpins, removing the showerhead electrode, replacing the gasket with a newgasket by locating projections on a lower surface of the gasket intorecesses in an upper face of a new or refurbished showerhead electrode,aligning locking pins of the new or refurbished showerhead electrodewith the axial bores in the backing plate, and rotating the cam locks toengage the heads of the locking pins.